High frequency pulse generation



May 17, 1960 M, GUTTLER HIGH FREQUENCY PULSE GENERATION Filed Oct. 24. 1957 forward by. a

lnvemor T Meinhord Giihier y Pa tent Agent back 4 A meter .Utlitc I generated with such systems.

e, 7 2,937,310 I v HIGH FR QUENCY PULSE GENERATION Giittler, Ulm (Danube), Germany, asslgnor to The present invention relates to a system for generating pulse-like oscillations of very high frequency by means of a master generator and a traveling wave tube operating as an amplifier and being'inserted in the circuit beyond the "generator.

Traveling wave tubes are electron tubes in which an electron beam is guided through the field of an electromagnetic wave at a velocity which is almost equal to the velocity of wave propagation,-said electromagnetic wave progressing along a delay line. During this operation, an interaction occurs between the electron beam and the wave, said interaction'being used to amplify the wave.

In traveling wave tubes, the input and the output are interconnected by means of this delay line or network, whereby a feed back path is obtained through which self excited oscillations may be produced. No precautions were taken in early prior art traveling wave tubes to prevent such self-excitation. Therefore, such tubes were limited .to a relatively low amplification because the theoretical maximum amplification could not be approached due to the occurrence of the self-excited oscillations.

- 'A substantial "increase in the amplification of traveling wave tubes has been obtained by providing high frequency attenuation means in the delay line path through which the electron beam passes and interacts with the high frequency field. This technique is conventional today. The portions of the waves reflected back in the direction to the input of the traveling wave tube by mis-matching at the output, or by any other disturbance, are attenuated on their way to the input of the tube by means of special attenuation means to such extent that the amplitude necessary to cause self-excitation is not reached, so that the tube will operate stably. The attenuation has always to be greater in the entire frequency range in which the traveling wave tubev amplifies than the amplification of this tube. The attenuation is smaller in the forward direction because the electron beam pushes, so to speak, the energy past the attenuation means. Thus, the theoretically possible amplification factor cannot be obtained in this manner. In other words, an attenuation in the forward direction has to be taken into account and a compromise has to be made between the maximal amplification factor and stability of operation of the tube.

It has been known per se to use traveling wave tubes for pulse operation. In the known systems of this kind, a high frequency control energy is fed to these tubes from a master generator, and the acceleration anode in the traveling wave tube, for example, is swept. The master generator may be a Klystron which can be tuned over a larger'range by mechanical means varying the resonator (Wobbel transmitter). It is likewise possible to connect in front of the circuit a wave generator, such as that sold under the registered trade name Carcinotron, which can be electronically tuned over a very large frequency range. High pulse energies can theoretically be However, in practice, the energy of. the pulse operation of traveling wave tubes is rather limited because of too great an amount of energy States Patent conversion into heat in the attenuation path, wherefro'm the dissipation of the heat is very diflicult.

It is an object .of the present invention to provide a traveling wave tube for pulse operation handling high pulse energies in such a manner that self-excitation in the tube is avoided. In this case, the conventional attenuation means in the tube can be omitted.

It is another object of the present invention to provide a traveling wave tube without high'frequency attenuation means within the delay line in which the electron beam interacts with the high frequency field, and to provide a master generator producing in the traveling wave tube beam current pulses of such length and distribution that the wave reflected along the delay line will encounter no electrons of the electron beam at the critical self-.

excitation point along the delay line.

Since the traveling wave tube accordingto the invention has no attenuation path, the tube c'annot'be overheated as mentioned in the introduction of this specifica tion, and, therefore, it is possible to generate in sucha tube pulses of high energy.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the I invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

In the drawings:

Figure 1 illustrates agraph showing the conditions in a traveling wave tube according to the invention;

Figure 2 shows schematically a circuit including the novel traveling'wave tube according to the invention;'

. Figure 3 is a modification of a circuit similar to Figure 2.

In the graph of Figure l, the length L of the delay line of the traveling Wave tube is shown on the abscissa, while the energy N of the oscillations amplified by this tube is plotted on the ordinate. The energy curve for the amplified oscillation is denoted by l. A portion of the amplified oscillation is reflected from the output A of the tube in the direction of the input E. The magnitude of this reflection depends upon the degree of mic-matching. The energy curve for the reflected wave is denoted by 2. This curve does not run exactly horizontally because there are small places of reflection on the delay line itself. Thus, the curve is slightly inclined in the direction of the input E. Small percentages of energy passing from the output A in the direction of the input E are constantly reflected in the direction of the arrow 3 at these reflection places on the delay line, i.e in the direction of the amplified oscillation. It may happen at one point along the delay line, for example, at the point P, that the energy reflected in the direction of the arrow 3 equals the energy of the amplified wave corresponding to the curve 1. The necessary amplitude required for selfexcitation is then reached at such point and the tube is in condition to initiate oscillations.

If the length and distribution of the current pulses emanating from the cathode of the traveling wave tube are selected according to the invention, i.e. if the current pulses are made so short that the wave returning on the delay line does not encounter any electrons when this Wave reaches the critical point of self-excitation, the traveling wave tube cannot stant oscillating.

The optimum pulse length is substantially dependent upon the amplitude of the reflected wave. If the reflection factor is small, the critical point is closer to the input end of the delay line, i.e., closer to the point E, and in this case, the wave can be caused to return nearer to the Patented May 17,1960;

3 input prior to self excitation. However, if the reflection factor is larger, a shorter pulse length should be selected.

If the master generator can be tuned over a large frequency range, larger reflections will generally be obtained near the extreme band limits. Therefore, it is recommended to make the length of the pulses adjustable. An automatically operating device may be provided for this purpose, said device adjusting the length of the pulse as a function of the ampliude of the reflected wave as measured by a ratio meter (Fig. 3).

If a greater mis-match between the output of the traveling wave tube and the load is present, the high frequency energy fedto the input of the delay line may cause standing waves on this line. These standing waves can be prevented according to a further development of the invention by providing a system for sweeping the supplied high frequency energy. This sweeping may be carried out in such a manner that it is synchronized with the pulse sequence controlled by the master generator connected to the traveling wave tube.

In the embodiment of the invention shown in Figure 2, only the essential circuit components are illustrated, i.e., a time delay line 10, a cathode 14, a control cylinder 13, a control anode 12 and an electron catcher 15. The electron beam emanating from the cathode 14 is denoted by 11. Input and output of the delay line 10 are denoted by E and A, respectively, as already indicated in Figure l. '17 and 18 are the voltage sources required for the operation of this traveling wave tube.

A pulse-like voltage is fed by the master generator 16 to the control anode 12. A sweep device 19 is provided at the input of the delay line. The high frequency voltage fed to the input of this delay line is swept in a pulselike manner by means of this sweep device 19.

An expert in this field will know how to calculate the delay time for the delay lines, the matching of the phase velocity of the wave to the velocity of the electrons, the determination of the potentials of the delay line and its length. Therefore it is unnecessary to give exact data for these parameters. To give an idea of the magnitudes, it may be said that the delay time over the total length of 4 the line will amount to about 0.5 [.L sec. in case of a potential of 1 volt and a 30 cm. lengthof the delay line.

I claim:

1. A high-frequency pulse amplifying circuit for amplifying a pulse from a generator generating a sequence of spaced pulses, comprising a travelling wave tube ampli-fier having an electron emitting cathode, having accelerating electrode means connected with said generator and accelerating said electrons to form a beam of spaced groups of electrons, the duration of each group being determined by the duration of each associated pulse, and said tube having an electron catcher and having a helical delay line between said catcher and said electrode means, the delay line having negligible attenuation to beam electrons reflected back along the line against the direction of travel of the beam; pulse length determining means coupled to said generator for selectively determining the lengths of the pulses in the sequence; a source of highfrequency voltage for energizing said delay line to produce therein a field for interaction with said electron beam; and sweep means connected to said source and to said delay line to modulate the high-frequency voltage energizing the line, the sweep means being connected to said generator and modulating the voltage according to said sequence of pulses.

2. In a circuit as set forth in claim 1, a ratio meter connected with said delay line and measuring the amplitude of the reflected stream of beam electrons; and said pulselength determining means being connected with said ratio meter and controlled thereby to vary the lengths of the pulses as a function of said amplitude.

References Cited in the file of this patent UNITED STATES PATENTS 2,392,380 Varian Jan. 8, 1946 2,653,271 Woodyard Sept. 22, 1953 2,702,370 Lerbs Feb. 15, 1955 2,704,350 Lerbs Mar. 15, 1955 2,770,722 Arams Nov. 13, 1956 2,813,996 Chodorow Nov. 19, 1957 

